Fresh-cut fruits and vegetables, with minimal processing and ready to eat, are the fastest growing segment of the produce market. Sulfite solutions were historically used to wash fruits, vegetables and mushrooms. Due to the detrimental effects of water and the undesirable effects of sulfites, minimally processed or ready to eat mushrooms with acceptable quality and shelf life for retail markets have not been achieved on a commercially viable basis.
Commercial production practices of growing mushrooms in straw-bedded horse manure compost covered with a fine layer of peat or other xe2x80x9ccasing materialxe2x80x9d yield harvested mushrooms with undesirable appearance and requires the consumer to wash the mushrooms prior to use. Mushrooms are typically harvested by hand leading to the introduction and spread of fluorescent Pseudomonads and other spoilage organisms that lead to accelerated decay and discoloration of the mushrooms.
Consumers identify whiteness and cleanliness of fresh mushrooms (Agaricus bisporus) as the main factors of quality. If an economical process could be developed to remove the casing material and compost from the surface of mushrooms while minimizing bacterial attack, the processors could create new markets and increase the sale of mushrooms. The consumer would prefer to purchase ready to use mushrooms that are free of such contaminants and have the opportunity to readily mix them with other food components. In the view of the grower/processor and end user, mushrooms would then join the category of minimally processed or fresh-cut produce and occupy the convenience section of the produce aisle.
The discoloration of mushrooms is due principally to enzymatic browning that is triggered when substrate and enzyme (tyrosinase or polyphenol oxidase) are allowed to mix. Tyrosinase, which occurs naturally at high levels in the cap cuticle or surface of the mushrooms, interacts with a number of phenolic substances that are also present in the cap. In healthy tissue, enzyme and substrate are segregated in separate subcellular compartments. Upon mechanical, bacteriological or physiological injury to the mushroom, enzyme and substrate are allowed to mix and subsequent discoloration occurs. Due to the fragile nature of mushrooms and susceptibility to attack by bacteria it would be highly desirable to develop a commercially viable protocol to minimize mechanical and bacterial damage to the mushroom tissue and thus indirectly inhibit enzymatic browning. It is further desirable to combine this with a preservation step that would directly inhibit enzymatic browning. It would be most efficient if such treatments could be a part of a washing process that would also remove undesirable particulate matter that clings to the mushroom cap surface after harvesting.
Traditionally, the surfaces of mushrooms have been washed with sulfite solutions to remove unwanted debris and bleach the mushrooms to a desired whiteness level. However, in 1986, the U.S. FDA banned the application of sulfite compounds on mushrooms due to allergic reactions experienced by asthmatic consumers when exposed to such compounds. Subsequent to the ban, numerous attempts have been made to identify alternative treatment compounds to sulfites. Although mushrooms treated with sulfite solutions exhibit a very desirable color at day 1 (post-treatment), there is little reduction in the surface microbial population. Hence, the beneficial effect of sulfite solutions on quality is short term. After only two to three days of refrigerated storage, bacterial decay of the sulfite treated mushrooms is evident. Growers accepted this trade off as sulfites are very cheap and the bleached appearance combined with the removal of undesirable debris yields an acceptable product for short periods of time. However, this short shelf life does not produce lasting results and is inadequate for retail distribution.
The banning of sulfite washes stimulated scientists to identify alternative systems and to extend the shelf life to meet the requirements for retail distribution. McConnell (1991) developed an aqueous preservative wash solution containing 10,000 ppm hydrogen peroxide and 1000 ppm calcium disodium EDTA. Hydrogen peroxide functions as a bactericide via oxidation injury to DNA and other cellular components. Ethylene diamine tetraacetic acid (EDTA) enhances the antimicrobial activity of hydrogen peroxide and reduces browning by sequestering copper, a cofactor required by tyrosinase. In 1994, Sapers modified McConnell""s protocol into a two stage process that utilized 10,000 ppm hydrogen peroxide in stage 1 and a combination of sodium erythorbate, cysteine and EDTA in stage two. Although these protocols were an improvement over the sulfite treatments, they proved expensive.
High pH solutions are known to be effective as anti-microbial treatments for mushrooms. Catalano and Knabel (1994) determined that increasing a solution wash to pH 11.0 caused at least a 3-log 10 reduction in the number of viable Salmonella cells within one hour of inoculation. Higher pH levels are especially effective against gram-negative organisms such as Pseudomonas, the predominant genus on mushrooms (Aubrey). In 1999, Beelman and Duncan incorporated the use of a high pH wash with an antibrowning solution in a two-stage process (U.S. Pat. No. 5,919,507). The Beelman-Duncan process used a high pH first stage as the antimicrobial treatment and a second stage of sodium erythorbate, calcium and EDTA to minimize enzymatic browning. While this process combined and improved the teaching of McConnell and Sapers, it narrowly focused on a two-stage sequence with limited chemical selection in each process step. Specifically, the Beelman-Duncan process limited the pH neutralizing step to include solutions of erythorbic acid and sodium erythorbate. In addition, Beelman applied the erythorbic solution immediately after the antimicrobial contacting step. This restrictive sequence resulted in the rapid degradation of the sodium erythorbate solution in the neutralization stage and the need for increased quantities of the anti-browning materials. Consequently, the Beelman process did not address the variability in raw material and proved too expensive to be adopted by the processors and did not achieve commercial viability.
While the science of preservation of fresh mushrooms has advanced from the days of sulfite wash solutions, there remains a need for an efficient and economical method for treating mushrooms that removes compost and casing material, reduces microbial activity, minimizes enzymatic browning and thereby improves appearance and increases shelf life of fresh whole and sliced mushrooms.
The present invention provides sulfite alternative compositions and a method for preserving mushrooms that is cost effective and provides adequate shelf life for retail distribution of the product. The method comprises the steps of contacting the mushrooms with an anti-microbial solution; rinsing the mushrooms with a neutralizing buffer solution; and treating the mushrooms with a browning inhibitor and a chelating agent.
Specifically, the instant invention provides a method for preserving mushrooms comprising the steps of contacting the mushrooms with an antimicrobial solution having a pH from about 10.5 to about 11.5; rinsing the mushrooms at least once with at least one aqueous pH neutralizing solution comprising organic acid and at least one salt of an organic acid substantially free from erythorbic acid and sodium erythorbate; and contacting the mushrooms at least once with at least one solution comprising a browning inhibitor and a chelating agent.
The invention further provides a three stage process that includes calcium and EDTA to minimize enzymatic browning in the third stage.